Variable speed photographic process printers that utilize CRT's for media exposure typically use linear deflection amplifiers to sweep the electron beam across the face of the CRT. A deflection cycle normally consists of a sweep, retrace, and hold period. During the sweep period, the CRT is unblanked and the beam is driven horizontally at a given vertical position while being modulated to expose the media. During the retrace period, the CRT is blanked and the beam is returned to a home position to wait for the start of the next sweep. Thus, the sweep and retrace periods are typically fixed time intervals. The beam is then held in the home position for the hold period until a new sweep is initiated. Therefore, the hold period is a variable time interval dependent on the print speed.
In typical prior art systems, the home position is at one end of the sweep. This requires the deflection amplifier to be held in a high current state in order to hold the beam at the end of the sweep. Thus the home position is a state of high power dissipation in the deflection amplifier. This causes a significant heat problem at low print speeds when the horizontal deflection amplifier must remain in a high power dissipation state for relatively long periods.
The sweep must normally be compensated for static and dynamic errors in order to cause the beam velocity across the CRT screen to be constant. A varying hold period affects the dynamics of the front end of the sweep period. Since the deflection amplifiers do not have infinite bandwidth, the beam dynamics at the start of each trace will be affected by the conditions prior to the start of the trace. A varying hold period makes these conditions variable causing beam dynamics to be difficult to control. The result is that beam velocity in the front end of the sweep changes with print speed and is particularly noticeable at high speeds when the hold time is short.
Another problem occurs when the beam is held at one end of the sweep during the hold time. Although the beam is blanked during the hold period, some leakage can occur. Since the beam is directed at an area of the CRT that can expose the print media, and since the hold period can be long, a small amount of leakage over this long period can cause unwanted exposure of the print media.
It is thus apparent that there is a need in the art for an improved system that reduces power dissipation in the deflection amplifier during the hold period. There is a further need in the art for a system to prevent leakage exposure of the print media during the hold period. There is a still further need for a system to eliminate dynamic deflection errors at the front end of each sweep. The present invention meets these needs.